1. Field of the Invention
The present invention relates to the field of electrophotgraphy, particularly liquid toned electrophtotoconductive imaging, and to a novel method of determining the charge per mass and liquid toner conductivity values contemporaneously for liquid toner in production.
2. Background of the Art
Electrophotography forms the technical basis for various well-known imaging processes, including photocopying and some forms of laser printing. Electrophotographic imaging processes typically involve the use of a reusable, light sensitive, temporary image receptor, known as a photoreceptor, in the process of producing an electrophotographic image on a final, permanent image receptor. A representative electrophotographic process involves a series of steps to produce an image on a receptor, including charging, exposure, development, transfer, fusing, and cleaning, and erasure.
Electrophotographic imaging is an established technology in a wide variety of imaging environments, not the least of which is desktop printing in black-and-white and full color. The technology advantageously uses liquid toner materials (also referred to as inks) in the production of high quality images. These liquid toners must be developed first on a lab scale and then be scaled up to mass production. Current liquid electrophotographic toner development processes require that multiple accurate measurements be taken for each new liquid toner formulation. Currently each test must be performed separately. Additionally, there are often impurities in the liquid toner that contribute to false or inaccurate analytical readings. The electrophysical nature of the electrophotographic process is itself well understood, as follows.
The prior art developing apparatus which is shown in FIG. 1 comprises a vessel 1 for a supply of dielectric fluid 2. The fluid 2, in turn, contains a dispersion of positively and negatively charged toner particles 3.
The vessel 1 further contains two spaced-apart electrodes 4 and 5 which dip into the supply of fluid 2. The electrode 4 is provided with at least one but preferably two or more suitable clamping elements 6 and 7 which can removably receive and hold a sheet-like carrier 8 of latent electrostatic images. For example, the sheet 8 which is shown in FIG. 1 can be inserted from above so that its lower edge rests on the clamp 7 and each of its lateral marginal portions is partially held by a discrete clamp 6. The latent image on the sheet 8 in the vessel 1 faces the electrode 5.
The electrode 4 is connected with a measuring resistor R1 by conductors 9 and 11. The other electrode 5 is connected with the resistor RI by a conductor 10. A further conductor 12 connects the conductors 9, 11 with the ground.
The toner particles 3 which are dispersed in the dielectric fluid 2 include negatively as well as positively charged particles, particularly at the start of a developing operation. The image bearing portions of the sheet 8 are negatively charged, and such negative charges are compensated for by positive charges on the adjacent portions of the electrode 4. The just mentioned positive charges on the electrode 4 are mirror images of negative (image) charges on the sheet 8. In the course of electrophoretic development, the negative image on the sheet 8 attracts positive toner particles 3 from the fluid 2, i.e., such positively charged toner particles 3 travel toward the adjacent surface of the clamped sheet 8 whereby the positive charges of the thus attracted particles 3 are neutralized by the negative charges on the image bearing portions of the sheet 8. The mirror symmetrical positive charges of the electrode 4 are thereby free to travel toward the electrode 5 to attract the negatively charged toner particles 3 in the fluid 2 and to cause them to advance in the fluid 2 toward and onto the electrode 5. These negatively charged toner particles 3 are neutralized when they reach the electrode 5.
It will be noted that, in the course of electrophoretic development process, there develops a current which flows between the electrodes 4, 5 and causes a voltage drop at the measuring resistor R1. The current flows until the entire electrostatic latent image on the sheet 8 is discharged as a result of deposition of toner particles 3 thereon. As shown in FIG. 2, a relatively high current i′0 develops in immediate response to insertion of a fresh electrostatically charged sheet 8 into the vessel 1, and such current rapidly decreases to a value i0 during the initial interval of time subsequent to insertion of a fresh sheet 8. The decrease of current from i0 to i1 is more gradual during the next-following interval of time (t1). The current i1 which flows after elapse of the interval T1 can be fairly approximately defined as follows:i(t)=i0 . . . e−t/96 .
The relationship with the neutralized charge on the sheet 8 can be established by ascertaining the area below the curve I of FIG. 2. The amount of charge (Q) can be ascertained by integration of the current function as follows:Q=integral[i0 . . . e−t/τ. . . dt]The extent to which the latent image on the sheet 8 is developed corresponds to the ratio of the area A (between the curve I and the abscissa and ordinate of FIG. 2) and the area A1 (which is bounded by the ordinate, abscissa up to the point t1, and the curve I). The area A is indicated by simple hatching, and the area A1 is indicated by criss-cross hatching. The areas A and A1 can be calculated by standard mathematic means. The ratio of the areas A1/A (i.e., the intensity or extent of development of the sheet 8) can be ascertained as is well understood in the art (e.g., U.S. Pat. No. 4,257,347).
As regards the ascertainable ratio of the momentary value i1 of the current flowing between the electrodes 4, 5 and the initial value i0 of such current, there exists the following relationship:i1/i0=1−A1/A=1−extent or degree of development.The curve I of FIG. 2 further shows that the actually achieved peak voltage at the start of the development closely approximates that which corresponds to the theoretically possible peak current value i′0 if one insures that, at the start of the developing operation, the developing fluid is supplied to the exposed surface of the sheet 8 as rapidly as possible and in the form of a laminar stream. For example, this can be achieved by resorting to a dipping device for the sheet 8 or by resorting to a fluid recirculating arrangement, e.g., an arrangement of the type shown in FIG. 1. The preceding equations are valid provided that, based on a uniform rinsing speed at the start of the developing operation, the ratio i′0/i0 remains at least substantially constant. That is, the toner density stays approximately the same and the voltage drops due to toner build up and current flow through the resistor is small.
FIG. 1 shows that, in order to ascertain the momentary value of the current which flows between the electrodes 4, 5 in the course of the electrophoretic developing operation, as well as to interrupt the developing operation when the desired degree or extent of development is reached, one can resort to the following circuit:
The conductor 10 which connects the electrode 5 with the measuring resistor R1 is further connected with the input of an amplifier V1 by means of a further conductor 13. The purpose of the amplifier V1 is to change the voltage (corresponding to current which is represented by the curve I of FIG. 2) to a voltage having a different (higher) amplitude. The two voltages are schematically shown to the left and above and to the right and below the amplifier V1 The amplitude-modified voltage is transmitted to one input of a comparator circuit K via conductor 14, and to a peak value storing circuit Sp via conductor 15. The circuit Sp stores the maximum voltage value (i.e., the initially transmitted voltage impulse), and its output transmits a constant voltage signal (schematically shown to the right of the circuit Sp) having an amplitude which corresponds to the peak value.
The voltage signal at the output of the circuit Sp is transmitted to an adjustable multiplying circuit V2, R2 and the intensity of such signal is reduced (as shown to the left of the component R2) to an extent corresponding to the desired degree of development of the sheet 8. The voltage signal (reference signal) of reduced intensity is transmitted to the left-hand input of the comparator circuit K via conductor 16. The conductors 17, 18 connect the output of the circuit Sp with the components V2, R2 of the multiplying circuit, and the conductor 19 connects the outputs of the components V2, R2.
The circuit K compares the momentary value (transmitted via conductor means 14) of the voltage at the resistor R1 with the somewhat reduced peak value (i.e., with the reference value) which is transmitted via conductor 16. Since the voltage at the resistor R1 is proportional to the current which flows between the electrodes 4 and 5, a relay S is energized at the exact moment when a certain preselected current i1 flows between the electrodes 4 and 5. The relay S is respectively connected to the ground (via conductor 12) and to the comparator circuit K by conductors 21 and 20. The energized relay S actuates a switch 22 at the time t.sub.1 so that the conductor 23 for the switch 22 can transmit a signal which starts the reversible motor 24M of a pump 24 in a first direction. The pump 24 then rapidly causes the fluid 2 to flow from the vessel 1 into a reservoir 27 and to thus complete the developing operation. The motor 24M is preferably a tandem motor, and it causes the pump 24 to convey the fluid 2 in the opposite direction (from the reservoir 27 into the vessel 1) when the switch 22 opens and the relay S closes a switch 29 in a conductor 31 connecting the relay S with the motor 24M. The reference characters 25, 26 respectively denote the conduits which connect the pump 24 with the vessel 1 and reservoir 27. The velocity with which the pump 24 can transfer the fluid 2 from the vessel 1 into the reservoir 27 is preferably sufficiently high so that the development of latent image on a sheet 8 which is clamped to the electrode 4 is terminated almost instantaneously, i.e., after elapse of the interval t.sub.1 following the start (t V0) of the developing operation.
The interval t1 determines the discussed ratio A1/A and hence the degree of development of the image on the sheet 8 in the vessel 1. To start the development of image on the next sheet 8, the relay S actuates a switch 28 which is mechanically or otherwise coupled to the aforementioned switch 29. The latter causes the conductor 31 to transmit a signal which starts the motor 24M in reverse, i.e., the fluid 2 is pumped from the reservoir 27 into the vessel 1. The switch 28 is connected with the relay S by a conductor 30 which forms part of the holding circuit of the relay. Such holding circuit is broken when the motor 24M is operated in reverse. The inflow of fluid into the vessel 1 is preferably rapid so that the development of image on the freshly introduced sheet 8 can begin practically instantaneously.
Because the electrophotographic toning process is so critically dependent in liquid toner electrophoresis and transfer, the ambient and varying properties of the liquid toner become very important, as well as the mere physical volume of toner remaining in the supply container. Processes and apparatus have therefore been developed to alert the user automatically when the properties and/or volume of the toner fail to meet requirements.
U.S. Pat. No. 4,577,948 (Lawson et al.) describes how changes occurring in the electrical conductivity of liquids are used to process image-wise exposed radiation-sensitive devices to measure the deterioration in effectiveness of the liquids. This deterioration is compensated for by varying the processing conditions, such as temperature, time, scrubbing action and processing liquid composition, in accordance with the change in conductivity. The reference uses an apparatus for processing image-wise exposed radiation sensitive plates which apparatus comprising (i) a container for processing liquid, (ii) a means of moving the plates along a path through the apparatus so that they are contacted by the processing liquid under given processing conditions, (iii) a means for measuring the electrical conductivity of the processing liquid and for producing an output signal in dependence on said conductivity, and (iv) a means of varying the processing conditions in dependence on said output signal, wherein said means of varying the processing conditions includes a variable speed motor for driving the plate moving means and controlled by said output signal so that the period of time for which the plates are in contact with the processing liquid is dependent on the conductivity.
Commonly assigned U.S. patent application Ser. No. 10/285,385, filed Oct. 31, 2002 (and which is herein incorporated by reference for its complete technical disclosure) describes a method for determining the concentration of toner solids present or remaining in any quantity of liquid solvent. One embodiment of the invention involves a series of steps. An electrical signal generator is electrically connected to a first electrode. A second electrode, attached or electrically connected to a detecting device, is positioned at a prescribed gap distance (e.g., between 0.005 inches and 0.250 inches) from the first electrode. The two electrodes are submerged in a liquid printing ink (in the practice of the invention in an electrophotographic imaging system, within the toner cartridge), maintaining the prescribed gap distance from one another. The signal generator then transmits an alternating current electrical signal (AC signal) or a direct current signal (DC signal) having a known amplitude to the first electrode. The direct current signal may be pulsed, and the receiving/signaling system may respond to the lack of pulses over a period of time to indicate depleted toner. The second electrode then receives any residual signal that is transmitted or propagates across the prescribed gap distance. The amplitude of the received signal is either detected at an acceptable intensity or determined to be absent or below the acceptable level, and a warning is generating based on whether the signal is received at the acceptable level or not received at an acceptable level (the unacceptable level including no signal received). Additionally, decisions may be made based on the amplitude of the received signal.
U.S. Pat. No. 6,154,620 (Hagiwara) describes a toner concentration measuring method and apparatus by which the concentration of toner in solvent can be detected with a simple construction without being influenced by a variation of the conductivity caused by a variation of the amount of ions in the solvent. A stepped dc voltage is applied from a high dc voltage generation section between a pair of electrodes placed in solvent, and very weak current which flows in a circuit formed from the pair of electrodes is measured by a current measuring section. The solvent between the pair of electrodes is replaced into an equivalent circuit, and a capacitance of the equivalent circuit is calculated in accordance with a circuit equation to determine the amount of ions in the solvent. Further, in accordance with a function expression wherein the ion amount and a resistance of the equivalent circuit are used as parameters, a toner concentration from which an influence of a variation of the amount of ions in the solvent is eliminated is determined.
U.S. Pat. No. 6,330,406 (Yamaguchi) describes a toner concentration detecting apparatus, which can detect a toner concentration of a developer without being influenced by ions, is provided. A first electrode and a second electrode are disposed face to face with a developer between the electrodes. First, a voltage of a first power supply is applied to the electrodes. After a designated time, by switching means, a voltage of a second power supply, whose polarity is different from the first power supply, is applied to the electrodes. By using a changing of current flowing between the electrodes caused by the difference between the transferring speed of toner particles and that of ions after switching the polarity of the power supply, a toner concentration calculating means calculates the toner concentration of the developer by using a table showing the relation between a peak value by the toner particles and the toner concentration. With this, the toner concentration can be calculated accurately. The toner concentration detecting apparatus, comprises: a first electrode and a second electrode which are disposed face to face with a developer between said electrodes; two power supplies, either one of which applies a voltage to said first electrode and said second electrode at one time; a switching means which switches polarity of said power supplies by switching from one power supply to the other power supply, after one power supply applied a voltage to said first electrode and said second electrode for a designated time; a detecting means which detects current flowing between said first electrode and said second electrode, at the time after applying voltage to said first electrode and said second electrode and after switching the polarity of said power supplies; and a calculating mean s which calculates a toner concentration of said developer, based on detected current values due to both ion and toner particles.
U.S. Pat. No. 6,535,700 (Caruthers) describes a toner developability sensor and method sense toner developability of liquid ink in an ink reservoir of a liquid ink image forming system. The toner developability sensor includes a power supply, a first electrode having at least one surface in contact with the liquid ink and connected to the power supply, and a second electrode spaced from the first electrode. When a potential difference is applied between the first and second electrodes, a developed toner layer is formed on the first electrode. A sensor senses at least one characteristic of the developed toner layer formed on the first electrode. The sensor detects characteristics of the developed toner layer that are directly related to the developability of the toner. The toner developability sensor that measures toner developability of a liquid ink contained in an ink tank, the liquid ink comprising toner particles suspended in a carrier medium, comprises: a power supply; a first electrode having at least one surface in fixed contact with the liquid ink in the ink tank and connected to the power supply; a second electrode disposed in the ink tank and having at least one surface in contact with the liquid ink and spaced from the first electrode, wherein, when a potential difference is applied between the first and second electrodes, a developed toner layer is formed on the first electrode; and a sensor that senses at least one characteristic of the developed toner layer formed on the at least one surface of the first electrode.
Alternative efficient methods of determination of operational parameters that are important for gauging the life and performance quality of liquid toner reserves are still desirable. This system may be used independently of imaging apparatus as an off-line testing system.